a round seal, often made of neoprene or some other compressible "rubber-like" compound, that is an industrial part.Looks like the part of a condom that unrolls before you actually unroll it. Can be used as a cock ringif you get the right size

"My buddy's a mechanic, I went thru his box of assorted O rings and found the size I can use as a cock ring. Nancii's gonna beg for mercy tonite !"